1. Technical Field
Embodiments of the present invention relate to a liquid ejecting head that causes liquid droplets to be ejected from nozzles by supplying a drive signal to a piezoelectric body, a driving method for driving ejection of liquid droplets from the liquid ejecting head. Embodiments of the present invention also related to a liquid ejecting apparatus that is provided with the liquid ejecting head, and a driving method for driving ejection of liquid droplets from the liquid ejecting apparatus.
2. Related Art
A liquid ejecting apparatus is an apparatus provided with a liquid ejecting head that is capable of ejecting various kinds of liquid as liquid droplets from nozzles. Examples of the liquid ejecting apparatus include, for example, image recording apparatuses (hereinafter, referred to as printers), such as ink jet recording apparatuses, that are provided with an ink jet recording head (hereinafter, referred to as a recording head), and perform recording by ejecting ink in liquid form as ink droplets from nozzles of the recording head. Further, in addition to the above, certain liquid ejecting apparatuses can be used to eject various types of liquids, such as coloring materials that are used in color filters for liquid crystal displays and the like, organic materials that are used in organic EL (Electro Luminescence) displays, and electrode materials that are used in electrode formation. Further, in some image recording apparatuses liquid ink is ejected from recording heads, and solutions of the respective color materials of R (Red), G (Green) and B (Blue) can be ejected from color material ejecting heads for display production apparatuses. In addition, a liquid electrode material can be ejected from electrode material ejecting heads for electrode formation apparatuses, and solutions of living organic matter can be ejected in living organic matter ejecting heads for chip production apparatuses.
A recording head, such as those mentioned above, can be provided with a piezoelectric element that causes pressure fluctuations in ink inside a pressure chamber. The piezoelectric element has a common electrode that is common to a plurality of piezoelectric elements and an individual electrode that is patterned individually in each piezoelectric element. A piezoelectric body layer (piezoelectric body film) is interposed between these electrodes, while a flexible cable electrically connects to terminals of the common electrode and the individual electrode.
When a drive signal (drive voltage) is supplied between the common electrode and the individual electrode through the flexible cable, an electrical field between the two electrodes is formed; the strength of the electrical field depending on a difference in voltage potential between the two electrodes. For example, normally, a constant potential is applied to the common electrode, and an oscillatory waveform is applied to the individual electrode. With this configuration, the piezoelectric element (piezoelectric body film) for example, bends and deforms depending on the intensity of the electrical field, which in turn causes a pressure fluctuation in ink inside the pressure chamber. This pressure fluctuation also results in the recording head ejecting ink droplets from the nozzles
In addition, the abovementioned drive signal may include a series of drive pulses with different waveforms. The drive pulses selectively applied to the piezoelectric element cause the recording head to eject ink droplets from the nozzles; a size (amount) of the ink droplets corresponding to the selectively applied drive pulses. For example, the drive signals that are shown in FIGS. 7A and 7B are provided with a large dot drive pulse PL that forms large dots on a recording medium (landing target), such as recording paper, by ejecting comparatively large ink droplets. The illustrated drive signal in FIGS. 7A and 7B also include a small dot drive pulse PS that forms small dots on the recording medium by ejecting comparatively small ink droplets. Both drive pulses PL and PS are provided with expansion elements p81 and p91 that cause a pressure chamber to expand by changing from an intermediate potential VC (a potential that is halfway between a maximum potential and a minimum potential) to expansion potentials VLL and VLS. Both drive pulses PL and PS are also provided with expansion retention elements p82 and p92 that retain the expanded pressure chamber for a set period of time by retaining the expansion potentials VLL and VLS and contraction elements p83 and p93 that cause the expanded pressure chamber to contract by changing from the expansion potentials VLL and VLS to contraction potentials VHL and VHS.
In addition, each drive pulse can be optimized for each recording head so that target ink droplets are ejected. More specifically, a difference in potential between the expansion potentials VLL and VLS and the contraction potentials VHL and VHS can be adjusted for each recording head. For example, in the drive signal that is shown as an example in FIG. 7A, a difference in potential (a maximum difference in potential) between the expansion potential VLS and the contraction potential VHS of the small dot drive pulse PS is set to be greater than a difference in potential (a maximum difference in potential) between the expansion potential VLL and the contraction potential VHL of the large dot drive pulse PL. On the other hand, in the drive signal that is shown as an example in FIG. 7B, a difference in potential (a maximum difference in potential) between an expansion potential VLL′ and a contraction potential VHL′ of a large dot drive pulse PL′ is set to be greater than a difference in potential (a maximum difference in potential) between an expansion potential VLS′ and a contraction potential VHS′ of a small dot drive pulse PS′. Additionally, the end terminal potentials of the large dot drive pulses PL and PL′ and the start terminal potentials of the small dot drive pulses PS and PS′ are connected and set to be uniform at the intermediate potential VC. In a printer that has this kind of drive signal, multi-gradation recording can be performed by selecting a drive pulse from the drive pulses in the drive signal, and changing the size (or number) of dots that are formed in a predetermined region (a pixel region) of a recording medium (a landing target), such as recording paper.
Given the above, an amount of displacement (an amount of deformation) of the piezoelectric body layer (the piezoelectric body) based upon an applied drive voltage (a difference in potential between the common electrode and the individual electrode) has a non-linear property (more specifically, a hysteretic property). In the piezoelectric properties of this kind of piezoelectric body layer, a linear region in which the piezoelectric properties have a linearity that is substantially close to a straight line is present in a certain region of the drive voltage. For example, in the piezoelectric properties of a piezoelectric body layer that is shown as an example in FIG. 6, a linear region L (a portion that is enclosed by a dashed line in FIG. 6) is present in the vicinity of where the drive voltage is 0. In this linear region L, a ratio of the amount of displacement with respect to the drive voltage is larger than non-linear regions other than the linear region L. Therefore, it may be desirable to adjust the drive signal so that the piezoelectric body is driven in the linear region L as often as possible.
On the other hand, there are circumstances in which the piezoelectric properties deviate from expected piezoelectric properties due to variation in the time of production and the like. When the piezoelectric properties of the piezoelectric body layer deviate, there is a concern that the ejecting properties of ink droplets ejected from the nozzles will deviate from the properties originally expected. Therefore, an apparatus that is configured to set the intermediate potential of the drive signal applied to the piezoelectric element to an optimum potential so as to suppress the influence of variations in the properties (the piezoelectric properties of the piezoelectric body layer) of the piezoelectric element of each recording head has been suggested (for example, refer to JP-A-2001-138551). That is, it is more convenient to adjust the intermediate potential than to adjust the potentials or inclinations of the constituent elements of the drive pulses, such as the drive pulses described in FIGS. 7A and 7B.
However, in a drive signal that has two or more pulses with differences in potential between the expansion potential and the contraction, there is a concern that adjusting the intermediate potential in the abovementioned manner will result in one drive pulse deviating from optimum conditions if another of the drive pulses is adjusted so as to match optimum conditions at which optimum ejection is performed. For example, in a case in which the piezoelectric body layer has piezoelectric properties such as those shown in FIG. 6, in the drive signal that is shown in FIG. 7A the expansion potential VLS of the small dot drive pulse PS matches a drive voltage V1, the contraction potential VHS matches a drive voltage V4, the expansion potential VLL of the large dot drive pulse PL matches a drive voltage V2 that is higher than the drive voltage V1, and the contraction potential VHL matches a drive voltage V3 that is lower than the drive voltage V4. With this in mind, in a case in which the potential of the large dot drive pulse PL is completely shifted to a low potential side to match the potential with a potential ideal for driving using the large dot drive pulse PL, that is, drive pulse balancing the amount of expansion and the amount of contraction of the pressure chamber, the intermediate potential VC is shifted to a low potential side. As a result, the small dot drive pulse PS is also completely shifted to a low potential side. This results in the expansion potential VLS of the small dot drive pulse PS being shifted to a region in which the inclination of the piezoelectric properties is smaller than the V1 (a region in which a ratio of the amount of displacement with respect to the drive voltage is small), and the contraction potential VHS being shifted to a region in which the inclination of the piezoelectric properties is larger than the V4 (a region in which a ratio of the amount of displacement with respect to the drive voltage is large). Therefore, driving due to the small dot drive pulse PS deviates from the ideal driving that is aimed for. That is, if the ejecting properties of ink droplets ejected from the nozzles using the large dot drive pulse PL are made to match intended properties, there is a concern that the ejecting properties of ink droplets ejected from the nozzles using the small dot drive pulse PS will deviate from the properties that are originally intended.
In addition, in the drive signal that is shown in FIG. 7B, the expansion potential VLL′ of the large dot drive pulse PL′ matches a drive voltage V1 and the contraction potential VHL′ matches a drive voltage V4. Further, in the drive signal that is shown in FIG. 7B, the expansion potential VLS′ of the small dot drive pulse PS′ matches a drive voltage V2 that is higher than the drive voltage V1 and the contraction potential VHS′ matches a drive voltage V3 that is lower than the drive voltage V4. With this in mind, in a case in which the potential of the small dot drive pulse PS′ is completely shifted to a low potential side to match the potential with a potential ideal for driving using the small dot drive pulse PS′, an intermediate potential VC′ is shifted to a low potential side. As a result, the large dot drive pulse PL′ is also completely shifted to a low potential side. This results in the expansion potential VLL′ of the large dot drive pulse PL′ being shifted to a region in which the inclination of the piezoelectric properties is smaller than the V1 (a region in which a ratio of the amount of displacement with respect to the drive voltage is small), and the contraction potential VHL′ being shifted to a region in which the inclination of the piezoelectric properties is larger than the V4 (a region in which a ratio of the amount of displacement with respect to the drive voltage is large). Therefore, driving due to the large dot drive pulse PL′ deviates from the ideal driving that is aimed for. That is, if the ejecting properties of ink droplets ejected from the nozzles using the small dot drive pulse PS′ are made to match intended properties, there is a concern that the ejecting properties of ink droplets ejected from the nozzles using the large dot drive pulse PL′ will deviate from the properties that are originally intended.
In this manner, in the related art, in a drive signal that has two or more different pulses, it is not possible to eject liquid droplets with optimal conditions that match the individual piezoelectric properties of the piezoelectric body layer in all of the pulses. In particular, in recent years, the thinning of piezoelectric body layers (piezoelectric bodies) has been progressing along with the miniaturization of recording heads. If the film thickness of the piezoelectric body layer is reduced, since the linear region L in the piezoelectric properties of the piezoelectric body layer becomes smaller, or in other words, since the non-linear region becomes larger, it becomes more likely that a range of the drive voltage that is used by other drive pulses will match the non-linear region. Therefore, deviation of ejecting properties such as that mentioned above becomes significant. In addition, as thinning of the piezoelectric body layer progresses, the amount of displacement of the piezoelectric body layer itself is reduced. Therefore, if the piezoelectric body layer (piezoelectric element) is driven in a region that is shifted from the linear region L in which the ratio of the amount of displacement with respect to the drive voltage is large, there is a concern that it will not be possible to apply a sufficient pressure fluctuation to the ink inside the pressure chamber.